Incubation of tissue slices or cell suspensions with highly radioactive amino acids permits a measure of the competition of an incorrect amino acid (e.g. H3-valine) for a correct amino acid (e.g. isoleucine). After a particular protein has been purified by techniques that should not separate proteins differing by a simple conservative substitution (such as electrophoretic methods), the protein is digested with trypsin which also should not distinguish between closely related proteins. If now a peptide that properly contains isoleucine but no valine is isolated and examined, it is found that a small amount of H3-valine is present in the isoleucine site. Comparison of its count-rate with that of valine in valine containing peptides is a measure of the error rate. For several sites in rabbit hemoglobin we find the rate is 1 per 3000. Other sites in rabbit hemoglobin are being examined and a rat liver ferritin system is being developed.